Rocks

Umber is a marine sediment associated with black smokers. It is perhaps best known as a pigment but I want to explain what it is as a rock type. It is important in Cyprus-type massive sulfide deposits and the examples shown below are also from Cyprus.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567856413283362
Umber is superbly preserved and very well exposed in Cyprus.

Black smokers are hot springs on the sea floor near spreading ridges. They emit hot water (more than 350 °C) that is mixed with various compounds, some of them metallic. Acidic water is rich in hydrogen sulfide and poor in oxygen. It is black in color because of sulfide precipitates, mostly pyrite.

Hot water usually also contains manganese, copper, and zinc in addition to iron. These sulfides precipitate and build chimney like structures on the sea floor. However, some of the material gets carried away into colder water where it will be precipitated as fine-grained sediment of iron and manganese oxides. This metalliferous fine-grained sediment, which may sometimes reach considerable thicknesses (tens of meters), is known as umber.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567925516192802
It sticks surprisingly strongly to the tongue if you are willing to conduct this test. It is an indication of high porosity of the rock.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567890406138434
Pieces of the rock are relatively lightweight although they now form, because of compaction, considerably thinner layer than originally.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567993256792210
The extent of compaction can be calculated because nearby are brown silicified rocks that are composed of umber if we subtract the silica.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567925659364082
The amount of umber in these rocks may be only one tenth of the pure layers in comparable volume of rocks. So it is clear that the silicification took place before the compaction.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567903718574466
Silica is hydrothermal: introduced to the rocks after the deposition.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567934089142882
Boulder of silicified umber.

http://picasaweb.google.com/107509377372007544953/Cyprus2#5737567990128259954
Outcrop of umber.

Overview and images of pegmatite as a rock type are here: Pegmatite

This pegmatite rock sample from Norway has a texture of a pegmatite (very coarse-grained igneous rock) but the composition is pretty unusual. There are small amount of pink K-feldspar that is the dominant component of granitic rocks but here much more dominant seems to be gray nepheline which is a feldspathoid group mineral. Feldspathoids form when there is not enough silica to form feldspars, let alone quartz which is entirely absent.

Similar rocks are often described as syenites but this is not correct because the feldspathoid/feldspar ratio is too large. This rock sample should be named foidolite according to the QAPF diagram which is commonly used to classify plutonic rocks. Even more precise term is nephelinolite because the dominant feldspathoid is nepheline.

Other notable minerals besides nepheline and K-feldspar are black hornblende, dark metallic gray magnetite, and yellowish brown wöhlerite. Wöhlerite is a pretty rare silicate mineral that occurs in silica undersaturated rocks like the one above.

http://picasaweb.google.com/107509377372007544953/Pegmatite#5783232943889936562
Pegmatitic igneous rock from Tredalen near Larvik. Gray is nepheline, yellowish brown is wöhlerite, black is hornblende, pink is K-feldspar, and gray is magnetite. The width of the specimen 14 cm.

Trachyte is a fine-grained and generally light-colored volcanic rock that usually has a rough surface to the touch which is the reason it was given that name (trachys is ‘rough’ in the Greek language). It was either Alexandre Brongniart or René Just Haüy who first defined that rock type (different sources go against each other in this question). They were French mineralogists who both left a lasting mark in the history of geology.

A sample from the type locality in Germany. Large sanidine phenocryst in the lower right. Width of sample 8 cm.

It is a feldspar-rich volcanic rock. It has been defined several ways in the past. Rocks that are nowadays known to us as andesites or rhyolites have both been named trachytes in the past. Trachyte is chemically between these two. We can say that the definition of this rock type is narrower now and it is not very abundant although its occurrences are widespread.

Trachyte usually contains lots of K-feldspar sanidine but plagioclase, anorthoclase, and sometimes feldspathoids are common as well. However, feldspathoid-bearing rocks are often named foid-trachytes (if we use QAPF diagram instead of TAS diagram) and trachytes that contain lots of plagioclase may be more properly described as trachyandesite (known also as latite). Volcanic rocks that are similar to this rock are rhyolite (contains more quartz), phonolite (more feldspathoids), trachyandesite (more plagioclase and dark minerals), and dacite (more quartz and plagioclase). See TAS diagram below to see how these rock types differ in their chemistry.

Trachyte is a volcanic equivalent of syenite. Syenite is a feldspar-rich plutonic rock which is similar to granite but lacks or contains very little quartz. Mafic minerals in trachyte are usually biotite, amphiboles (hornblende or arfvedsonite), and pyroxenes (diopside, augite, aegirine). Aegirine and arfvedsonite occur in the rock when it is rich in alkali metals (compositionally close to phonolite). Trachyte that is unusually rich in silica (>20%) is named trachydacite although in the QAPF diagram we would name it a rhyolite instead. So, these classification schemes are really not flawless.

Trachyte is probably a product of magmatic differentiation. Its parent magma was perhaps basaltic but it evolved (its composition became enriched in alkalies and silica) by the removal of mafic minerals. It may be associated with phonolite, latite, rhyolite, etc. which means that the same volcano has extruded magmas with slightly different composition. Trachyte is not necessarily volcanic in the strict sense. It may also form underground but still relatively close to the surface because its grain size is fine. Coarse-grained rocks with a trachytic composition are know as syenites as said before. Magma with a trachytic composition may also solidify as obsidian or pumice.

http://picasaweb.google.com/107509377372007544953/Rocks#5841867258084946754
TAS diagram with the field of trachyte annotated.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841862800933337890
A sample with anorthoclase phenocrysts from Gran Canaria. Width of sample is 6 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841862992250770210
Fine-grained sample from Tenerife. Width of sample is 15 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841873832641184146
An outcrop of trachyte in Tenerife. Width of view is about 10 meters.

Overview and images of pegmatite as a rock type are here: Pegmatite

Pegmatites are coarse-grained igneous rocks. Here is a sample from Norway which is composed of three minerals: spessartine (manganese-bearing garnet group mineral), muscovite (light-colored common mica), and white feldspar.

The sample was originally described simply as “spessartine”. It is obvious why, such a nice specimen which shows so many large garnet crystals is by no means common and it is easy to neglect the other components. I like individual minerals as well but perhaps even more I like the assemblages of minerals (which we call rocks) and the stories associated with them. That’s why I try to understand what could be the other minerals to see the broader picture.

The only instrument I have used to analyse the sample is my pair of eyes but I have no doubt that the greenish gray flaky mineral is muscovite. Muscovite is a very common mica, especially in pegmatites. The white mineral is a little trickier. It looks like feldspar but which one? White feldspar like this could be Na-rich plagioclase (albite) but K-feldspars (orthoclase, for example) may be very similar. I do not believe it could be Ca-rich plagioclase because muscovite is usually associated with felsic rocks which host K-feldspars and sodic plagioclase.

I think it is plagioclase because on the other side of the rock I saw that the crystal is in some places composed of narrow lamellae which reflect light differently when looked at a certain angle. I try to look for that if I suspect that the mineral might be plagioclase. It probably isn’t pure albite because these lamellae are not present in near-pure end-members of the plagioclase series. So, it could be oligoclase for example which is the next mineral in the plagioclase series after albite. In albite, up to 10% of the sodium is replaced with calcium. The percentage is 10…30 in oligoclase.

Overview and images of pegmatite as a rock type are here: Pegmatite

Seiland is an island in Northern Norway where really spectacular pegmatitic rocks are found.

Pegmatites are often mineralogically unusual but this one is probably the weirdest I have seen. There seems to be only two minerals: biotite and zircon.

Both biotite and zircon are common magmatic minerals but what is really striking is the size of the zircon crystals. Zircon usually occurs in very small grains because there is normally not enough zirconium in the magma to grow large crystals.

In this case things seem to be different. I don’t know what led to the formation of such a mineral assemblage but one thing that’s sure is that mineral collectors definitely love samples like this. Similar looking rocks are probably widely distributed in the collections worldwide because these rocks were mined there between 1980-1988. The rock body that contains large zircon crystals embedded in biotite is nepheline syenite dike (about 8 meters wide at the widest and several kilometers long).

Serpentinite is a metamorphic rock that is mostly composed of serpentine group minerals. Serpentine group minerals antigorite, lizardite, and chrysotile are produced by the hydrous alteration of ultramafic rocks. These are igneous rocks that are composed of olivine and pyroxene (peridotite, pyroxenite). Serpentine group minerals occur less commonly in some olivine-bearing marbles (ophicalcite) and kimberlites, but these rocks are normally not considered to be serpentinites.

http://picasaweb.google.com/107509377372007544953/Peridotite#5779439247917903922
Serpentinite from the Troodos ophiolite in Cyprus.

Serpentinites form as a result of serpentinization. These are chemical reactions which convert anhydrous ferromagnesian silicate minerals (pyroxene, olivine) into hydrous silicate minerals (serpentine) plus some other possibilities like brucite and magnetite. Brucite forms if the precursor rocks are rich in magnesium (dunite, for example). Magnetite forms if there is enough iron present (pyroxenite). Usually serpentinite contains iron in the form of magnetite which gives dark color to serpentinites.

Serpentinite is probably very widespread rock deep below, but not nearly as common in the upper parts of the crust. Here it occurs mostly where ultramafic rocks occur (ophiolite complexes). Serpentine minerals along with other green-colored alteration minerals (talc and chlorite) are still pretty widely distributed because olivine and pyroxenes are readily available in many places. Serpentinites has been important sources of asbestos, but nowadays the use of asbestos has diminished considerably because of health concerns.

These concerns may actually be greatly exaggerated because there is not much good evidence that chrysotile (serpentine asbestos which makes up more than 90% of all asbestos mined) poses any significant risks to our lungs. The danger associated with asbestos comes mostly form amphibole asbestos minerals, but it is hopeless to assume that the fearmongering media would take the trouble and educate themselves and their readers about the different types of asbestos minerals. So, we continue to see dangers in harmless amounts of chrysotile asbestos in floor tiles while we neglect or happily tolerate far more important health risks like fast food and lack of physical exercise (which kills millions every year).

http://picasaweb.google.com/107509377372007544953/Rocks#5850409733080359362
Serpentinite where the dominant serpentine mineral is fibrous chrysotile. The sample is from the Sayan Mountains in Siberia. Width of sample is 8 cm.

Serpentinite (green) with white magnesite from Norway. Serpentine and magnesite may occur together if there were enough carbon dioxide available during the metamorphism to form magnesium carbonate magnesite. Width of sample 24 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127849628250690
Serpentine-rich skarn sample with hedenbergite (iron-rich Ca-clinopyroxene). Serpentine in skarn may form if the carbonate host rock is dolomite that provides lots of magnesium to form magnesium-rich silicates like serpentine minerals. Tapuli, Sweden. Width of sample 13 cm.

Overview and images of eclogite as a rock type are here: Eclogite

This text is full of strange geological terminology. If you don’t understand terms like omphacite, pyroxene, ultra-high pressure, metamorphic, actinolite, facies, etc. then it may be somewhat painful to read. You are warned!

Here is a rock which at first raises no questions. It definitely has to be a nice sample of eclogite. Eclogites are metamorphic rocks with roughly basaltic composition that form at extremely high pressure. They are usually easily identifiable because the colorful components (red garnet and green pyroxene) form very beautiful and easily recognizable mineral assemblage.

But then I stared at the label (the sample belongs to a geology museum) and saw there only one word describing the sample: smaragdite. I wasn’t sure what it means but it didn’t shatter my belief in my eclogite hypetheses. I just thought that maybe ‘smaragdite’ is an alternative (commercial) name for eclogite. ‘Smaragd’ means emerald in many languages, including my own. Later I learned that smaragdite is actually a variety of actinolite (actinolite is a member of amphibole group minerals). So, the label actually describes only the green part of the rock.

This is still very important information (I assume the identification is correct) because it changes few things. The rock indeed is or at least was eclogite because smaragdite forms at the expense of omphacite (the green pyroxene in eclogite). So, we are talking here about retrograde metamorphic reactions which are associated with pressure release. But then I thought that perhaps we shouldn’t call that rock eclogite anymore. Eclogites are ultra-high pressure rocks, they contain no hydrous phases. But that’s exactly what actinolite is (like all other amphiboles). And isn’t it mandatory for eclogite to contain omphacite? What is it then? Metaeclogite? That surely does sound strange. Is it possible to metamorphose something that is already highest grade metamorphic rock? Does retrograde metamorphism count as a valid reason to add the prefix meta- to the metamorphic rock name? I doubt it.

The mineral assemblage seems to refer to the amphibolite facies. Perhaps it should be called garnet amphibolite (however, it lacks plagioclase, essential constituent of amphibolite) or garnet actinolite amphibolite? Okay, I actually know very well that it won’t go through. The rock does look like eclogite and it at least was true eclogite before. That is probably enough to end all further speculations about the proper naming. Geology isn’t exact science but what I want to stress here is that perhaps purely theoretically we shouldn’t name it eclogite anymore.

By the way, what is the white stuff there? Could it be sillimanite? I know that sillimanite is not a stable phase at such pressure (stable Al-silicate is kyanite there which sometimes occurs in eclogites) but maybe kyanite also metamorphosed to sillimanite later as pressure released because the mineral does look much more like sillimanite than kyanite to me.

So, I’d be happy to read your comments if you happen to know more about that particular rock or the general naming principles of metamorphic rocks.

Calc-silicate rocks are very fascinating for me. They are beautiful and their composition is very interesting. Some time ago I wrote a post about a skarn from Italy that is composed of calcite, grossular, and augite.

Today I want to demonstrate a similar rock sample but this time there seems to be entirely different set of minerals: wollastonite, diopside, and andradite. But it only seems because augite is strongly related to diopside. Grossular and andradite are both garnet group minerals that are compositionally similar and they both occur in similar rocks. Wollastonite is also calc-silicate (calcium-bearing silicate) that is almost exclusively found in this type of rocks.

Overview and images of pegmatite as a rock type are here: Pegmatite

I have recently used several examples of pegmatite to illustrate my posts. The reason is very simple — pegmatites contain many unusual but still important minerals and they tend to be coarse-grained which makes them excellent teaching materials.

But pegmatites are not always full of exotic stuff like lepidolite, topaz, and beryl. They are often compositionally common granites. The only difference is the grain size. Today I want to demonstrate a sample from Germany which seems to be ordinary granite but contains large biotite flakes with pink and white feldspar and gray quartz. These are the most common constituents of granite.